A was for Austin, but now B is for Bedford

PDU, you need someone to donate a metal Roller to your Mens Shed..!

PDU, Google metal slitters.

PDU, I once gave a mate a hand to build an aluminium "plate" boat. To curve the front of the hull, we used ratchet straps to bend the sheet over the frames, but keeping the straps directly over the frame so as to not bend the sheet out of shape, if I've explained that properly. We put a strap over each second frame so we could then weld the intermediate frame.

Might be confusion of metal terminology. Zincalume is the high tensile stuff used for roofing and will fight you all the way. Zincanneal is used for bodywork as it can be shaped and takes paint well. Some sheet metals have a grain so bend better one way than the other.

I think you'll find if you try and bend the metal over you frame as it is the metal directly against the frame will stretch ever so slightly to accomodate the bend while the metal in the middle without the pressure of the free against it won't stretch and therefore try to take a shorter route leaving you with a slight hollow between each support. I'd suggest trying something like MDF of plywood to make a curved surface over the frame that you can then use to bend your steel around.

PDU I think IHScout has the right idea.  A mate of mine uses a lot of 3mm plywood under things he makes.  I do not think MDF would curve around that radius.  Mate has a supply of that as well but only uses it on flat panels.The plywood will give insulation and if you are lucky and can get some adhesive between the plywood and the zincanneal, will help prevent drumming as well.  I would start at the TOP, and rivet or tek screw it down.I know it will be more difficult but once you have fastened the top edge, gravity will be your friend and the weight of the sheet hanging down will make it easier to carefully smooth it down over the curve of the plywood. Cargo straps are a good idea once the top edge is secured.

I would love to come and help you but I live in another State and am about 11 hours drive from you.  (aren't I lucky)

Thanks all. Zincalume or zincanneal, no idea mammoth . . . but it does bend smoothly when I drag it over the curve IHScout so plywood shouldn't be necessary . . . with or without Morris's help. I considered starting at the top but if there is any runout/wandering (with the sheet metal) life becomes difficult down at the bottom. By starting at the bottom any over hang or alignment issues can be fixed at the top by corrective trimming - even possible to bend the top edge down into the lift up roof hole with a few of cobbadog's 'light taps with a planning hammer' would allow the use of rivets horizontally AND vertically!    

Something thatg might help is cutting the sheets to lennnnngth so they are a little shortert han the 2.4 lengths you are starting and allowing for an overlap of 50mm half the 3.8 overall length plus 25mm x2 will give you the right length. Shorter lengths must be easier to handle plus may give you more options to find rollers 2000mm wide to do the job. I was in a sheet metal shop te other day buying my checker plate and they had a heap of toys we all would want at our disposal and from memory the rollers there would be close to being 2000mm wide so tey are around.

I don't think it has been suggested yet, but i have curved sheet steel in a panbrake (or folder) by making ever so slight bends every 5mm or so, to create a curve. 
I would imagine access to a panbrake, or folder, locally would be considerably more likely than a set of rollers of the required width.

I had considered multiple small folds 180wannabe BUT it looks as though it will pull around the curve okay as is, and using ratchet straps sounds like the go-to controllable solution.  

The sheets will be trimmed to overlap at the "centre" uprights cobbadog and nobody in this neck of the woods has any rollers remotely long enough. 

Bear with me, it will take me all weekend to sort out the lower section . . .